In the past, difficulties have been experienced in various attempts to brown or crispen foods in a microwave oven. A microwave oven heats foods differently from a conventional oven. Generally speaking, food substances are heated in proportion to their tendency to absorb microwave radiation, which may result in considerably different heating patterns from those which exist in a conventional oven. Also, microwave radiation penetrates into most foods in a way which results in considerably different heating patterns from those which would otherwise be present in a conventional oven. In most cases, microwave energy will heat foods faster than in a conventional oven. For example, a food substance which might require 30 minutes to properly "cook" in a conventional oven, may take only 3 or 4 minutes to "cook" in a microwave oven. In a conventional oven, the oven atmosphere is heated to relatively high temperatures to transfer heat to the food surface resulting in the surface always being the hottest area in the food. In a microwave oven, the oven atmosphere is generally not heated; the food itself heats and transfers heat to the surrounding air resulting in the food surface being cooler than the interior. These differences significantly affect one's ability to brown or crispen a surface of a food in a microwave oven.
Many attempts have been made to brown or crispen the surface of a food in a microwave oven. One such attempt has involved the use of packaging components called susceptors. Suitable susceptors may contain microwave absorbing coatings which are deposited upon a microwave transparent support layer. These susceptors heat when exposed to microwave radiation. A susceptor may achieve temperatures high enough to brown or crispen the surface of a number of food products. The susceptor may be placed in close proximity to, or in direct contact with, the surface of the food product. A typical, commercially available susceptor contains a thin film of vacuum deposited aluminum on polyester which is then adhesively laminated to paper or board.
The use of susceptors, however, has resulted in additional problems. Available susceptors typically do not heat uniformly. As a result, such susceptors may not crispen or brown the food substance uniformly. For example, the outer region of a susceptor may become much hotter during microwave irradiation as compared to the center region of the susceptor. As a result, the outer portion of the food substance may tend to become brown or crisp, but the center portion will not do so without overcooking the outer portion. This is a particular problem in food substances which have large surface areas, for example, the baked crust of a large frozen pizza. When a susceptor pad is used, for example, to crispen several fish sticks arranged side by side on the susceptor, microwave heating may typically result in fish sticks on the ends of the susceptor which are crisp, but fish sticks in the center of the susceptor pad may not be adequately crisp.
An additional problem of heating foods using susceptors is the lack of control of the heating profile across the susceptor surface. It is often desirable to adjust the amount of heat output in sections of a susceptor to accommodate different food characteristics. This is a particular problem when two or more foods with varying browning/crisping requirements are placed in conjunction with a common susceptor. When heated, one food's contact surface may become overcooked while an adjacent food's contact surface may remain soggy.
When a food substance is cooked by microwave radiation, particular attention must be paid to the overall energy balance achieved during the heating process. If an attempt is made to simply increase the strength of the microwave radiation or cooking time in an effort to brown or crispen a particular area of the surface of the food substance, this may result in overheating or overcooking of other surfaces and/or the interior of the food substance itself. In other words, if one seeks to achieve browning or crisping of the center area of a pizza crust by simply increasing the heating time or by increasing the strength of the microwave radiation, the likely result would be an overcooking of the outer surface of the pizza and/or an overcooking of the pizza toppings.
Heating foods in a microwave oven, particularly where susceptors are employed, usually involves a complex balancing of energy which is absorbed throughout the food substance. Although the use of susceptors has resulted in some improvement in the browning or crisping of food substances in a microwave oven, the need has existed for solving the problem of susceptors which do not heat uniformly. Some means for browning or crispening food products uniformly with a susceptor has been needed. The need has further existed for some means to achieve uniform browning and crispening without disturbing the complex energy balance necessary to properly heat all portions of the food substance. Also, the need has existed to differentially brown or crispen various types of food products.
Examples of attempts to achieve crispening of food products is shown in U.S. Pat. No. 4,267,420, issued to Brastad, and U.S. Pat. No. 4,230,924, issued to Brastad et al. Brastad attempted to produce flexible wrapping material which was wrapped completely around a fish stick to brown the surface of the fish stick. However, Brastad did not address the problem of nonuniform crispening of the food surface. Brastad did not disclose how to compensate for nonuniform heating caused by the flexible wrapping material.
Another example is U.S. Pat. No. 4,641,005, issued to Oscar E. Seiferth. A thin film susceptor is disclosed for heating foods. However, Seiferth did not address the problem of nonuniform heating of the susceptor surface. Seiferth did not disclose how to compensate for food loads or how to compensate for susceptor preferential edge heating.
It will be apparent from the above discussion that prior art attempts to achieve crispening of the surface of a food substance in a microwave oven have not been altogether satisfactory. The use of susceptors has often resulted in nonuniform crispening of the food surface and undesirable nonuniform heating patterns.